The invention relates to a pipe lay vessel with a reel for carrying a length of pipe and first drive means for controlling rotation of said reel, a substantially vertical guide structure comprising at its upper end a deflection member for deflecting the pipe from a first trajectory extending between the reel and the guide structure to a second substantially vertical trajectory, and a clamping member for carrying the vertical pipe section having second drive means for feeding the pipe along the vertical trajectory.
Such a vessel is known from WO-96/35902 in which a pipe is spooled from a storage reel to a bending shoe mounted on a derrick structure that is situated above a moon pool. The vertical pipe section is passed via a 3-point straightening assembly into the water. The known J-lay vessel is particularly suitable for pipe laying in deep water. It is known to suspend the vertical pipe section from a clamping device or tensioner, which can be driven or braked to feed the vertical pipe section towards the seabed. The reel supplying the pipe is also driven or braked in a controlled manner.
Variations in pipe speed along the first trajectory between the reel and the deflection member will occur because the pipe coming from the reel will have a varying speed, even at constant reel rotation since the pipe is not always evenly spooled on the reel and the diameter of the unspooled pipe on the reel varies during the unspooling process. Also during start up and stopping and during emergency crash stop situations, speed variations of the pipe will occur along the first trajectory and along the vertical trajectory where the pipe is fed through the tensioners. Speed variations may also be caused by plasticity of the pipe and partial straightening upon unspooling. Because of the varying tension along the first trajectory between the reel and the deflection member, sagging of the pipe or overload of structural elements that guide the pipe may occur.
It is therefore an object of the present invention to provide a pipe lay system with which the tension of the pipe along the first trajectory can be maintained substantially constant. It is a further object of the present invention to provide a system in which differences in speed of the pipe section coming from the reel and the pipe section fed along the vertical trajectory can be compensated.
It is again a further object of the present invention to provide a pipe lay system having a relatively simple and reliable compensator system. Again, it is a further object of the present invention to provide a pipe lay system in which a pipe straightener is provided which is not affected by the tension compensation system and to provide a pipe straightener which can accommodate pipes of different diameter.
Thereto the pipe lay system of the present invention is characterised in that that a movable tension compensator is provided at the first pipe trajectory for lengthening or shortening the first pipe trajectory to maintain a substantially constant tension of the pipe along the first trajectory during unwinding of the reel
By displacing the tension compensator, the first pipe trajectory can be lengthened or shortened to compensate for variations in speed due to non uniform winding of the pipe and varying diameter of the spooled pipe and to compensate for sagging of the pipe due to its own weight and length variations upon unspooling in view of occurring plasticity and partial straightening of the pipe. By the movable tension compensator, the tension in the pipe along the first trajectory (the back tension) can be maintained substantially constant.
In one embodiment of the pipe lay system according to the present invention, a detector is placed near the tension compensator for measuring its position relative to an equilibrium position. A control unit receives position signals from the detector and supplies a control signal to the drive means of the reel and/or to drive means of the clamping member for changing the pipe laying speed such that the tension compensator is at least substantially returned to its equilibrium position. When the tension in the pipe section between the reel and the deflection member becomes too large, the tension compensator will move such that the first trajectory is shortened. The control unit may either increase the unwind speed of the reel, such that the tension compensator is moved back to its equilibrium position or may lower the speed at which the pipe is fed along the vertical section. Similarly, when sagging of the pipe occurs, the tension compensator is moved such that the length of the first trajectory is increased. Decreasing the unspooling speed of the reel or increasing the pipe speed along the vertical section will result in the tension compensator returning to its equilibrium position.
Detecting the position of the tension compensator may be carried out optically or by encoders measuring a hinge angle of the compensator. The latter can occur when the tension compensator comprises a curved arm extending transversely to the vertical guide structure and being hingingly attached in a hinge point, remote from the guide structure. The end of the curved arm opposite the hinge point may be attached to a fluid cylinder, which is extended or compressed when the tension compensator moves away from its equilibrium position. In this case, measuring the pressure in said cylinder or measuring its stroke will provide a signal indicative of the deflection of the tension compensator from its equilibrium position.
In one embodiment according to the present invention, a pipe straightener element is provided on the hinging arm of the tension compensator, the pipe straightener element having a curved surface and a radius of curvature substantially originating in the hinge point. In this way the point of departure of the vertical pipe section from the upper pipe straightener element to the lower two pipe straightener elements can remain constant such that the pipe straightening moments in the pipe straightener remain unaffected by movement of the tension compensator.
Analysis of the shape of the pipe before entering the pipe straightener has taught the applicant that the curvature of the pipe in the region of the straightener varies for relatively large diameter pipes (diameters of about 40 cm or more) whereas Correlatively small diameter pipes (diameters of 10 cm or less) the curvature upon entry into the straightener is more or less constant. Especially for larger diameter pipes it is advantageous that at least one of the pipe straightener elements comprises a closed track around two rollers, at least one adjustable roller being placed between the two rollers for altering the width of the track. By controlling the track width of the pipe straightener, for instance by hydraulic cylinders or by screw spindles, a proper straightening for each pipe diameter can be obtained in combination with the movable tension compensator according to the present invention.